Cardiac rhythm management devices are implantable devices that provide electrical stimulation to selected chambers of the heart in order to treat disorders of cardiac rhythm. A pacemaker, for example, is a cardiac rhythm management device that paces the heart with timed pacing pulses. The most common condition for which pacemakers are used is in the treatment of bradycardia, where the ventricular rate is too slow. Atrio-ventricular conduction defects (i.e., AV block) that are permanent or intermittent and sick sinus syndrome represent the most common causes of bradycardia for which permanent pacing may be indicated. If functioning properly, the pacemaker makes up for the heart's inability to pace itself at an appropriate rhythm in order to meet metabolic demand by enforcing a minimum heart rate and/or artificially restoring AV conduction. Other cardiac rhythm management devices are designed to detect atrial and/or ventricular tachyarrhythmias and deliver electrical stimulation in order to terminate the tachyarrhythmia in the form of a cardioversion/defibrillation shock or anti-tachycardia pacing. Certain combination devices may incorporate all of the above functionalities.
Cardiac rhythm management devices such as described above monitor the electrical activity of heart via one or more sensing channels so that pacing pulses or defibrillation shocks can be delivered appropriately. Such sensing channels are made up of implanted leads which have electrodes disposed internally near the heart, which leads may also be used for delivering pacing pulses or defibrillation shocks. Such implanted leads, however, may also act as antennas for extraneous electromagnetic fields, referred to as electromagnetic interference (EMI). For example, theft prevention systems commonly employ high-strength magnetic fields to detect the presence of a magnetic tag placed on retail merchandise. Most of these systems modulate or pulse the magnetic field in such a way that the repetition rate falls within the passband of cardiac sensing amplifiers. When these fields are coupled to the implanted leads of a cardiac rhythm management device, signals are produced in the device's sensing channels which may be misinterpreted as cardiac electrical activity, thus causing inappropriate inhibition of pacing and/or inappropriate delivery of defibrillation shocks.